Almost none of the 51 stone artefacts selected from Birka burials (Fig. 28) were observed on all sides via SEM (Table 12; Figs. 46 and 47). Not all of them have the ‘classic’ four faces; in fact, in addition to thin and flat (two-sided) artefacts, three- and five-sided touchstones also occur (both made of banded silt). Due to time constraints, chemical microanalyses were often only conducted on a single face. As on the studied parts of the artefacts, traces of metals will surely also have survived on the faces that were not investigated. Although surprises can never be ruled out, the analysis of these additional sides would probably have resulted in an increase in quantity of the same data as has already been obtained. As is likewise the case with touchstones from Vendel and Tuna in Alsike (Swedish Uppland: Ježek 2014; 2016), the frequency of the preserved streaks on several touchstones from Birka is exceptional on a European scale. However, it is not possible to establish whether this phenomenon is a reflection (most probably) of the intensity of metal testing, or of the methods used for cleaning off previous streaks in the Early Middle Ages, or (less probably) of the influence of the chemical composition of the local soil, or whether it is the result of post-excavation care (objects removed from the ground at the above sites in Swedish Uppland during the late nineteenth century are all deposited in the same museum).
The summary below has no quantitative objective. Setting aside the numerous circumstances influencing the preservation of metal traces on touchstones, this was probably because, when observing various streaks on a single touchstone, recurrent or highly similar results of chemical microanalyses were not recorded. On the other hand, the traces that were recorded could come from the repeated testing of the same metal artefacts, whereas others could be the remnants of a single streak preserved in fragmentary form. As a result of the non-homogeneity of the tested metals, spot analyses conducted today on various parts of the same streak sometimes produce different results. Certain elements contained in the streak need not show up in the spot analysis, and therefore it is necessary to analyse it at various points. In the case of unusual results, the spot analyses were repeated on various parts of the streak; however, they are not repeated in the individual sections of Table 12. In brief, any type of mathematical, let alone
statistical, method for evaluating the data as presented would be misguided. Moreover, chemical issues also play an important role (see pp. 117-121). Some metals do not form compounds in alloys. For example, if silver dominates an alloy of silver and copper, crystals of silver grow first, followed by an allotriomorphic mix of Ag and Cu with a eutectic composition. Lead present in alloys has an even greater influence on the eutectic substance. The overview provided below must therefore be taken as a report on results obtained by SEM, not a detailed report on the products of early medieval metallurgy.
Silver often occurs among the analysed streaks on Birka touchstones (on 33
specimens observed in SEM, i.e. 65 % of our assemblage), either in ‘pure’ form or with a small accompaniment of other metal, usually gold or copper, and rarely tin or zinc. In the case of tin its occurrence is not regarded as relevant as to whether this involved, for example, bronze added as an adulterant into coinage metal, since the chemical composition of many streaks is consistent with high quality coins. In a small number of cases, the ratios of individual metal components are approximately balanced. One streak belongs to a metal composed of silver and copper with an addition of gold (grave 605B), whereas another streak displays similar amounts of gold, copper and silver (grave 495). In another case the concentration of silver and lead is virtually balanced, with the metal also containing copper (grave 795).
The presence of a trace amount of chlorine in cases involving silver is presumed to be the result of the effect of the reaction of the silver streaks with airborne chlorides, with NaCl from human sweat, or hydrogen sulphide resulting from the decomposition of organic material, with hydrochloric acid concentrated in the digestive juices of higher animals, or the effects of micro-organisms. Silver and metals containing silver are often accompanied by sulphur, a phenomenon interpreted as the result of the natural reaction occurring when silver comes into contact with an organic material (the tarnishing or ‘blackening’ of silver). Silver reacts with sulphur from the air to produce sulphides and sulphates (e.g. acanthite Ag2S), and the creation of
sulphides is particularly intensive in the presence of hydrogen sulphide (H2S), which is produced during the decomposition of proteins.
In certain cases at Birka, iodine accompanies silver with a copper content (graves 86 b, 220, 495, 496, 605B, 795, 831, 1035 – the longest specimen), in other cases ‘pure’ silver (graves 145, 495, 524, 557, 795, 831) and sometimes ‘pure’ copper (376A). A conclusive explanation for the presence of iodine has not yet been found. A proposal seeking to clarify the same results on touchstones from Tuna in Alsike, Sweden, likewise exclusively in combination with silver and/or copper in ores containing iodine (Ježek 2014, 424), is not credible. Halides could not survive the smelting process even in trace amounts; the iodine potentially contained in the ores would evaporate during metallurgical processing. The results do not involve the remnants of testing minerals such as iodargyrite AgI, miersite (Ag,Cu)I, or marshite CuI; the atomic weights of iodine are too low in the relevant analyses. According to Milan Holub (pers. comm.), a possible source for the iodine in the documented streaks of silver, copper and their alloys is seaweed – or an ash from it. This ash could have served for cleaning tarnished non-ferrous metals and could also have lent a welcome
99 metals on touchstones from birka and elsewhere
patina to silver artefacts (silver iodide turns bright yellow when exposed to light). However, the presence of iodine was also recorded in the chemical microanalysis of ninth-century wire, again in a metal dominated by silver and copper, although only in one of four analyses conducted on various areas of an artefact discovered at one of the Great Moravian centres (Czech Republic, Galuška 2013, fig. 130).
In addition to numerous traces of other metals, one touchstone (of banded silt from grave 56) also bears streaks of silver with a relatively high proportion of mercury and a small amount of copper and iodine.45 The stable phase of the
Ag – Cu – Hg system already melts at 400-500 oC. Mercury distils at higher
temperatures, whereas crystals of the Ag – Cu system do not fully melt until temperatures of 950-1100 oC are reached. The mercury could not have survived
the intentional production of the metal of which a streak is preserved on the touchstone. The same is true for iodine. Our attention turns to the extraordinary deposits in Swedish Uppland that were being exploited as early as the Middle Ages (Falun, Sala, Garpenberg: for refs., see Ježek 2016). Approximately 100 ppm Hg and 200 ppm Ag is reported in the sphalerite concentrate in Sala. The contents of Ag in galena at the location are variable; galena contains a variety of inclusions and exsolutions of silver minerals including tetrahedrite, which often contains silver and mercury (freibergite, schwatzite). In addition to other ores and minerals, paraschachnerite Ag3Hg2, schachnerite Ag1.1Hg0.9, amalgam
Ag – Hg, kongsbergite AgHg, Hg-tetraedrite CuHg12Sb3S13, eugenite Ag11Hg2, etc., also occur in the Sala mine (Zakrzewski & Burke 1987, with refs.); for the occurrence of mercury in the silver context, see also Table 1 (prestige Iron Age barbarian burial). In any case, the presence of iodine in the metal under discussion on the touchstone from grave 56 indicates that the streak is not a result of recent contamination.
Streaks of gold were recorded on 17 selected stone artefacts from Birka burials (i.e. 33 % of the assemblage observed in SEM). Gold most frequently occurs in ‘pure’ form or with an accompaniment of both silver and copper; less frequent is gold with a content of silver and gold with a content of copper. The vast majority of gold streaks documented on touchstones from the analysed assemblage are in excess of 20 carats. These do not include at least two streaks with an exceptional character extending beyond the Birka assemblage. A streak from a metal composed of gold with nickel and a trace amount of zinc was documented on the above-mentioned stone from grave 660, whereas a streak of a metal with an approximately 50/50 composition of gold and tin, with a content of copper, is preserved on the stone from elite grave 624. A minor amount of chlorine appears in one streak of gold (grave 715), as in a similar case from Vendel (Ježek 2016, tab. 2). The most probable explanation can be found in the decay of organic materials, which also releases chlorides – among other compounds. There are, however, many other possible explanations, ranging from gold cementation to gold refinement.
45 This situation, different from the evidence of the amalgamation of brass recorded at Hedeby, necessi- tates the retraction of premature footnote 34 in Ježek & Holub (2014, 200), with my apologies.
Lead is generally the most common metal found on early medieval touchstones from the north-eastern part of Europe, or at least in Sweden and Poland. It occurs also on c. 80 % of the touchstones in the investigated assemblage from Birka. Of the dozens of documented lead streaks, approximately the same number are of ‘pure’ lead and lead containing copper (in rare cases also zinc: brass?). Lead with a content of tin appears in a small number of cases, in others also of copper. The small number of cases of metals composed of approximately the same amount of lead and copper have been mentioned above; just as infrequent are metals with the same amount of lead and tin. Also previously mentioned was a streak of a metal with a roughly identical content of lead and silver and with a content of copper (grave 795). Lead also occurs as an accompaniment to both tin and copper (in a small number of cases together with silver), and in a rare instance as an admixture (?) in brass with a dominant component of zinc (grave 561). In several cases, lead is accompanied by chlorine (for similar results from both Tuna in Alsike and Vendel, see Ježek 2014, tabs. 1 and 2; 2016, tabs. 1-3). Lead reacts strongly with certain organic acids occurring during the decomposition of organic material and also readily reacts with chloride ions under certain conditions.
Streaks of tin have been recorded less often on touchstones from Birka. Although ‘pure’ tin is most common, in some cases it is accompanied by copper. Tin with a content of lead has been recorded in only a small number of cases, in some instances with trace amounts of copper, exceptionally also with zinc (or brass). Of greater interest are streaks on the stone from the above-mentioned grave 624: tin repeatedly dominates in the streaks of a metal composed of gold and copper with a minor amount of zinc (lead is present in one case); in another case, mentioned above, an analysis of the same touchstone revealed approximately the same representation of tin and gold with a content of copper. In rare cases, tin also appears as an accompaniment of silver or copper (or brass), in several cases combined. Tin creates organic compounds that can bind sulphur from hydrogen sulphide from rotting and decaying proteins. If the tin is close to chloride ions, it quickly corrodes; the chlorine probably comes from halite (e.g. in human sweat).
There are dozens of copper streaks in the studied assemblage. Copper appears in ‘pure’ form, as well as with various concentrations of lead and zinc, in rare cases with tin and silver. These also include streaks of copper with a content of zinc and tin (graves 752, 949), of zinc and nickel (grave 750 – the slate specimen), of silver and lead (795), of lead and zinc (949), etc. Far more often than as the dominant or only metal, copper occurs in the analysed assemblage (often in trace amounts) as an accompaniment of lead, silver, tin, gold or of metals composed of some of them.
In several cases copper is accompanied by chlorine, in one case (grave 60) by chlorine and sulphur. The presence of chlorine in the streaks of copper or metals containing copper can be explained by corrosive processes accompanied by the formation of mixed oxy-hydroxy-chloride Cu2Cl(OH)3.
Like silver, copper reacts with sulphur to produce sulphides and sulphates (e.g. verdigris), which can be expected in an environment rich in hydrogen
101 metals on touchstones from birka and elsewhere
sulphide, i.e. especially in inhumation burials. However, a minor content of sulphur in the streaks of copper can be also explained by the imperfect separation of sulphur during the smelting of copper ore (Ježek 2014, 425). In cases accompanied by sulphur, it is sometimes difficult to establish whether documented streaks represent tests of intentional products or of the raw ore itself. They represent intentional alloys in cases where the share of copper is approximately the same as the share of lead (graves 573 and 605B, both touchstones with silver rings). This is also true for similar streaks without the presence of sulphur (graves 145, 955, 973), and probably for numerous compounds of copper and lead (or conversely) without sulphur, where one of the metals is subdominant or marginal.
Zinc appears relatively often as subdominant in various streaks, especially
of lead and tin, as at Birka. Zinc (5 %) also occurs in a metal with a dominant share of gold (77 %) and nickel (18 %: the stone from grave 660; see p. 109). The ratio of zinc to copper is typically up to one-third (33 %) in the traces of brass on specimens from Birka. Although this figure is in line with current literature on archaeometallurgy, the number of documented streaks on touchstones from Birka is uncommonly high. More surprising is that Birka does not belong to the sites where streaks of brass with an even higher share of zinc – up to 40 % – were observed. Such results have been recorded on touchstones from other early medieval European sites (e.g. Ježek 2013b, tab. 3; Ježek & Zavřel 2013, 125, tab. 3; Ježek & Holub 2014, tab. 1; Ježek
et al. 2010, 68, tab. 1; 2013, tab. 1); contemporary brass artefacts with a
similar share of zinc are also known (for burial finds, e.g., Frána & Tomková 2005, 321, tab. 4; Děd 2012, 292). Zinc gives a golden hue to brass; as yet unanswered is whether the aim of such alloys was to imitate a precious metal or whether it was a random by-product of classic brass production technology that caught the attention of touchstone users with its colour. Mentioned above is a unique (not only at Birka) metal composed of zinc, copper and lead in which the share of zinc is one half (grave 561), and a metal composed of zinc, copper (or brass) and silver in which the share of zinc is again nearly one half (grave 605B; for another example, see Table 3). If this metal was intentionally produced – for example, by dissolving silver in brass rich in zinc – it could have successfully imitated high-quality silver. However, the presence of zinc in this and other metals need not necessarily be connected solely with handling brass.
An even higher concentration of zinc in a copper alloy has been recorded during the analysis of prehistoric and early medieval stone artefacts, although this is rare. On the specimens from the La Tène oppida of Staré Hradisko and Třísov, Czech Republic (Table 3), the proportion of zinc in the clearly intentional streaks reaches up to 60-82 %. A metal of this type is consistent with a two-component brass in the gamma phase, which is known for its hardness and brittleness. Although it has no use from a contemporary perspective, its distinctive golden colour could have been a good reason for its intentional production (at temperatures above 820 oC), and especially for testing (on
Fig. 29. Bruszczewo, Greater Poland: (A) selected (fragments of) touchstones from the tenth- to eleventh-century settle- ment context (Poznań Archaeological Museum); (B) inv. no. 1976:248, right (horizontal, including the cluster) a streak of copper, left (vertical) a streak of nickel; (C) inv. no. 1960:968, streak of zinc; (D) spectrum of the streak of nickel on the touchstone inv. no. 1976:248. For complete results, see Table 7.
B C
3 cm
A
103 metals on touchstones from birka and elsewhere
touchstone46 from burial A31 in cemetery 116 at Helgö, Uppland, Sweden. A
further three (of 17) typical stone artefacts from Helgö selected for chemical microanalysis bear traces of ‘pure’ zinc (inv. nos. 26481 A3947, 30249 F16 A25,
30710 F4 A37), sometimes accompanied by chlorine, as is also the case with the touchstones from Birka graves 47, 86 b, 220 and 573. The discussion of chemically identical and apparently intentional streaks preserved on touchstones from prestige boat graves at Vendel and Tuna in Alsike (both also Swedish Uppland) concluded that there is no reason to doubt their early medieval origin, or rather, that the appearance of chlorine supports an interpretation of this kind (Ježek 2014; 2016). Although seemingly a somewhat unwelcome topic in archaeometallurgical literature (with rare exceptions: see Rehren 1996; Rehren in Fellmann 1999; Rehren & Martinón-Torres 2009; Nováček 2004; with refs.), evidence for zinc processing in European classical antiquity and the Middle Ages includes a small rolled sheet from the Athenian Agora, an inscribed tablet from a Gallo-Roman sanctuary near Berne, Switzerland, fragments of a Romanesque liturgical vessel from Pipe Aston, England, etc. (see Ježek 2016).48
Even after analysing hundreds of touchstones from a large part of Europe, linear streaks of zinc on touchstones are rare, unlike grains (in dozens of micrometres: e.g. Tables 3, 7 and 10; Ježek & Zavřel 2013, 124-125). From the Slavic environment, distinct streaks of zinc were observed on one (fragment of ) touchstone from the tenth- to eleventh-century settlement located near the fortress at Bruszczewo, in Greater Poland (Fig. 29; Table 7), and on a touchstone (again, a fragment missing the most important part) from the twelfth- thirteenth-century, ordinary, sunken house in the village of Dřetovice, Czech Republic (Fig. 30).49 Sphalerite, as a mineralogical rarity, occurs in pelosiderite
concretions in the vicinity of Dřetovice (for more about the local use of this unusual source of raw iron in the eleventh to thirteenth centuries AD, based on both contemporary documentary and archaeological sources, from the vicinity of the eponymous Knovíz, see Anderle et al. 2000, 54-63).
However, the majority of the observed linear, clearly intentional, streaks of zinc have so far been recorded on early medieval touchstones from Swedish Uppland (again, a matter of our logistic possibilities). Nevertheless, even here,
46 Swedish History Museum, Stockholm, inv. no. 30249 F10 (A31).
47 Streaks of silver, copper, lead and tin with an admixture of lead and copper are preserved on this touchstone. The inv. nos. are those of the Swedish History Museum, Stockholm.
48 The remarkable early medieval hoard from Rostock-Dierkow, Germany, contains a touchstone (now lost) and five bars, three of which should be made of an ‘alloy of copper and zinc […] with a significant share of zinc’, whereas the other two bars ‘were probably made of zinc’ (Warnke 1992/93, 204; trans- lated from German). In fact, three brass bars are composed of 73-78 % Cu and 20-26 % Zn, with a marginal presence of tin and lead; the other two bars are made of tin, with a marginal amount of copper. I thank Detlef Jantzen and Lorenz Bartel for enabling the analyses in the Landesamt für Kultur und Denkmalpflege Mecklenburg-Vorpommern in Schwerin, and Jan Mařík for the analysis using a portable XRF spectrometer. Incidentally, even the title of the cited paper is at fault: there is no reason to ruminate on the hoard of a goldsmith, jeweller, etc.
49 I thank Kateřina Opicová for the chance to study the object from Dřetovice, where two further medie- val ‘whetstones’ and dozens of kilos of iron slag were also recorded in the 1950s (Opicová 2016, 50-54).